Wireless communication systems operate using information modulated onto a radio frequency (RF) carrier to form an RF signal that is subsequently transmitted over a channel. Often the RF signals are formed from in-phase (I) and quadrature-phase (Q) signal components. The RF signal is received by a radio receiver, and the modulated information contained in the RF signal is demodulated to recover the desired information. Demodulation strategies are well known in the art; however receiver topologies generally introduce noise or distortion into the demodulated signal. The amount of noise or distortion depends on the specifics of the receiver configuration and the type of information modulated into the signal.
Direct Conversion Receivers (DCRs) are incorporated into many communication systems as the receiver topology of choice because of their relatively low cost, small size, and operational flexibility over a wide range of channel spacing (i.e., the frequency difference between adjacent channels used to transmit data). A receiver is referred to as a DCR if the down conversion mixer is sourced by a Local Oscillator (LO) having a frequency approximately equal to the desired RF carrier frequency, thereby converting the received RF signal to a baseband signal. A DCR receiver may also include receiver systems where the frequency difference between the LO and the desired RF carrier signal is less than several hundred kiloHertz (kHz) which may also be called a Very Low Intermediate Frequency (VLIF) configuration for the DCR topology.
However, because of the direct conversion to baseband, DCRs are more sensitive to interferers (interference signals) than receivers that modulate to baseband in intermediate steps. For example, DCRs are sensitive to constant envelope (both TDMA and continuous) interferers that cause DC offsets at baseband. DCRs are also sensitive to interferers having non-constant envelopes, which cause distortion due to the second-order term of the polynomial that models the DCR nonlinear response. DCRs are designed to have a high second order intercept point to increase rejection of the second-order distortion terms. It is thus desirable to design a DCR and reception method that provides improved rejection of the interferers.